Optical scanner and image formation apparatus

ABSTRACT

An optical system includes three optical systems. The first has a coupling lens. The second includes a lens having a positive power in a vertical scanning direction and forms the light flux into a line image extending in the horizontal scanning direction on a deflector. The third includes a first lens having a positive power in the horizontal scanning direction, and a second lens having a positive power in the vertical scanning direction. Lateral magnification in the horizontal scanning direction is set larger than that in the vertical scanning direction. Temperature near the first lens is maintained higher than that near the second lens.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to an optical scanner used for laser-beamprinters (LBP), regular paper facsimile systems (PPF), digital copyingmachines and the like, and an image formation apparatus that employssuch an optical scanner.

2) Description of the Related Art

High density, such as 1200 to 2400 dpi (dots per inch) has recently beenrequired as the scanning density in optical scanners. In order toachieve high density in optical scanning, it is necessary to decreasethe beam diameter of optical beams condensed on a surface to be scanned.While a requirement for decreasing the beam diameter increases as theoptical scanner has a higher density, it is also required to manufacturethe optical scanner at a low cost. In order to deal with thisrequirement for low cost, a lens made of resin is often used for thescanning lenses. However, in the resin lens, image-formingmisregistration is large due to a temperature change, and it isdifficult to decrease the beam diameter.

As an apparatus in which such image formation misregistration issuppressed, there are known apparatus disclosed in U.S. Pat. No.2,736,984 (Publication 1), and U.S. Pat. No. 2,804,647 (Publication 2).

The apparatus in the Publication 1 has following components. That is, acollimator comprising a semiconductor laser, a collimator lens and aretaining member which fixes and retains these components, and ascanning image formation optical system which forms an image of a lightflux from the collimator deflected by a deflection unit on aphotosensitive material. By optimizing a coefficient of linearexpansion, a refractive index or the like of the collimator lens, thescanning image formation optical system and the retaining member, theimage formation misregistration of the whole optical system is reduced.

The apparatus in the Publication 2 comprises following components. Thatis, a first image formation optical system which forms an image of lightfrom a light source linearly, and a second image formation opticalsystem which allows optical beams deflected by a deflection apparatushaving a deflecting reflective surface to condense on a surface to bescanned, at an image formation position of the first image formationoptical system. By using a resin lens (plastic lens) having a negativepower for the first image formation optical system, an image formationmisregistration which occurs in the second image formation opticalsystem is cancelled, to thereby reduce the image formationmisregistration of the whole optical system.

As the optical scanning speed of the optical scanner increases, itbecomes necessary to rotate a deflector such as a polygon scanner at ahigh speed. Therefore, the temperature in the vicinity of the deflectordiffers from the temperature in the vicinity of a position away from thedeflector, due to heat generated by the deflector which rotates at ahigh speed, and hence the temperature distribution in the opticalscanner, that is, in an optical housing, becomes non-uniform. As aresult, it is necessary to take measures for suppressing image-formingmisregistration, on the assumption that the temperature distribution inthe optical scanner is non-uniform.

However, in any of the Publications 1 and 2, it is not described norsuggested that image formation misregistration is suppressed on theassumption that the temperature distribution in the optical scanner(that is, in the optical housing) is non-uniform. In other words, theoptical scanners described in the respective Publications 1 and 2 do notsuppress image formation misregistration, on the assumption that thetemperature distribution in the optical housing is non-uniform, due tothe heat generated by the deflector which rotates at a high speed. Thus,the optical scanners described in the respective Publications 1 and 2have a problem in that even if measures as described in each publicationare taken, the beam diameter of the optical beam and a pitch deflectionof a vertical scanning beam pitch of multi-beam are deteriorated.

In the optical scanner in the Publication 2, as described above, a resinlens having a negative power is used for the first image formationoptical system, so that an image formation misregistration which occursin the second image formation optical system is cancelled. However, inorder to improve the correction result of the image formationmisregistration, it is necessary to set the negative power of the resinlens large. Thereby, machining of the resin lens becomes difficult, andthere is another problem in that the wave front aberration isdeteriorated.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide an opticalscanner that can reduce changes of beam diameter due to a temperaturechange and a pitch deflection of a vertical scanning beam pitch ofmulti-beam.

It is a second object of the present invention to provide an opticalscanner that can reduce changes of beam diameter due to a temperaturechange and a pitch deflection of a vertical scanning beam pitch ofmulti-beam, as well as realizes a smaller diameter by obtainingexcellent wave front aberration.

It is a third object of the present invention to provide an imageformation apparatus that can output a stable high-quality imageexcellent in granularity, resolution and gradient, by using the opticalscanner that can reduce changes of beam diameter due to a temperaturechange and a pitch deflection of a vertical scanning beam pitch ofmulti-beam, as well as realizing a smaller diameter.

The optical scanner according to one aspect of the present inventioncomprises a first optical system having a coupling lens that couples alight flux from a light source; a second optical system that forms thelight flux into a line image extending in the horizontal scanningdirection, on a deflector, the second optical system including a glasslens having a positive power in a vertical scanning direction, or aresin lens having a positive power in the vertical scanning direction; athird optical system having a scanning image formation device thatcondenses the light flux deflected by the deflector as an optical beamspot on a surface to be scanned, the third optical system including afirst optical element made of resin, having a positive power in thehorizontal scanning direction, and a second optical element made ofresin, having a positive power in the vertical scanning direction; and atemperature distribution generation unit which controls atmospherictemperature T1 near the first resin optical element, and atmospherictemperature T2 near the second resin optical element such that T1>T2.The an absolute value of a lateral magnification in the horizontalscanning direction of an optical system, which includes the firstoptical system, the second optical system, and the third optical system,is set larger than an absolute value of a lateral magnification in thevertical scanning direction of the optical system.

The optical scanner according to another aspect of the present inventioncomprises a first optical system having a coupling lens that couples alight flux from a light source; a second optical system that forms thelight flux into a line image extending in the horizontal scanningdirection, on a deflector, the second optical system including a glasslens having a positive power in a vertical scanning direction and aresin lens having a negative power in the vertical scanning direction; athird optical system having a scanning image formation device thatcondenses the light flux deflected by the deflector as an optical beamspot on a surface to be scanned, the third optical system including afirst optical element made of resin, having a positive power in thehorizontal scanning direction, and a second optical element made ofresin, having a positive power in the vertical scanning direction; and atemperature distribution generation unit which controls atmospherictemperature T1 near the first optical element, and atmospherictemperature T2 near the second optical element such that T1>T2. The anabsolute value of a lateral magnification in the horizontal scanningdirection of an optical system, which includes the first optical system,the second optical system, and the third optical system, is set largerthan an absolute value of a lateral magnification in the verticalscanning direction of the optical system.

The image formation apparatus according to still another aspect of thepresent invention employs the optical scanner according to the presentinvention.

These and other objects, features and advantages of the presentinvention are specifically set forth in or will become apparent from thefollowing detailed descriptions of the invention when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the configuration of an optical scanneraccording to a first embodiment,

FIG. 2 is a cross sectional view along the line A1—A1 in FIG. 1,

FIG. 3 explains an optical system in the optical scanner shown in FIG.1,

FIG. 4 is a sectional view which shows a surface perpendicular to theoptical axis (a surface perpendicular to the face of paper) in theoptical system shown in FIG. 3,

FIG. 5 is a block diagram of the configuration of another opticalscanner according to the first embodiment,

FIG. 6 is a sectional view which shows a surface perpendicular to theoptical axis (a surface perpendicular to the face of paper) in theoptical system shown in FIG. 5,

FIG. 7 is an enlarged diagram of each lens constituting the secondoptical system shown in FIG. 5 are enlarged, and

FIG. 8 is a block diagram of the configuration of the main part of animage formation apparatus according to a second embodiment.

DETAILED DESCRIPTIONS

The embodiments of the optical scanner and the image formation apparatusaccording to the present invention will be explained below in detailwhile referring to the accompanying drawings.

FIG. 1 is a block diagram of the configuration of the optical scanneraccording to a first embodiment of the present invention. FIG. 2 is across sectional view along the line A1—A1 in FIG. 1.

As shown in FIG. 1, the light flux emitted from a light source 1 iscoupled in a desired state of light flux by a coupling lens 2. Here, thelight flux is coupled substantially in a parallel light flux. As for thelight source 1, there can be used a semiconductor laser array having asemiconductor laser and a plurality of light emitting points, or amulti-beam using a beam combining-type light source which combinesoptical beams from the semiconductor laser by a prism or the like. Thecoupling lens 2 is an a spherical lens having a single lens. The wavefront aberration by the coupling lens 2 alone is favorably corrected.The light flux emitted from the coupling lens 2 enters into a lens 3having a power in the vertical scanning direction, and is condensedsubstantially linearly, extending in the horizontal scanning directionin the vicinity of a deflecting reflective surface of a deflector 4.

The light flux deflected by the deflecting reflective surface of thedeflector 4 transmits through a first scanning lens 5 having a power inthe horizontal scanning direction and a second scanning lens 6 having apower in the vertical scanning direction, while being deflectedisometrically, with uniform rotation of the deflector 4. The opticalpath of this transmitting light flux is bent by a bending mirror 7,while the curvature of field respectively and optical characteristicssuch as fθ characteristic in the horizontal scanning direction and inthe vertical scanning direction are being corrected via the firstscanning lens 5 and the second scanning lens 6, and an image is formedon a surface to be scanned 9 a of an image supporting body 9, via awindow 8, as shown in FIG. 2.

The beams enter into a mirror 10 prior to optical scanning, and arecondensed into a photodetector 12 by a lens 11. Write timing in theoptical scanning is determined based on the output of the photodetector12. The optical components (e.g., optical elements) denoted by referencenumerals 1 to 7 are housed in a housing 13. The housing 13 is coveredwith a cover 14, and the inside is substantially in a closed state.

Reference numerals 20A and 20B in FIG. 1 denote ribs for preventing heatgenerated by the deflector 4 from being transmitted towards the secondscanning lens 6. The ribs 20A and 20B do not hinder the heattransmission completely, but is formed in such a manner that thetemperature T1 in an atmosphere in the vicinity of the first scanninglens 5 becomes higher than the temperature T2 in an atmosphere in thevicinity of the second scanning lens 6. In other words, the ribs 20A and20B generate a temperature distribution so as to satisfy the relationtemperature T1>temperature T2. These ribs 20A and 20B correspond to thetemperature distribution generation unit described in the claims.

The optical system in the optical scanner according to the firstembodiment will be explained below, with reference to FIG. 3. FIG. 3 isa diagram which explains the optical system in the optical scanner shownin FIG. 1. In FIG. 3, from the light source 1 to the coupling lens 2 aretermed as a first optical system 21, the lens 3 is termed as a secondoptical system 22, and an optical system formed of the first and secondscanning lenses 5 and 6 is termed as a third optical system 23. Thefirst scanning lens 5 corresponds to a first optical element made ofresin, and the second scanning lens 6 corresponds to a second opticalelement made of resin.

As well known, the deflector 4 rotates at a high speed and becomes aheat source. In the Example 1, however, the ribs 20A and 20B areprovided in the vicinity of the first scanning lens 5, as shown in FIG.1, so as to positively utilize the heat source, thereby generating atemperature distribution which realizes the relation: temperatureT1>temperature T2.

Specifically, in the first to the third optical systems, the followingtemperature distributions are obtained. That is,

Temperature T01 in the first optical system 21=45° C.,

Temperature T02 in the second optical system 22=45° C.,

Temperature T1 in the vicinity of the first scanning lens 5=45° C., and

Temperature T2 in the vicinity of the second scanning lens 6=35° C.

The expression of the shape of the lens surface of a lens whichconstitutes a scanning image formation device, that is, the scanningoptical system depends on the following polynomial.

The surface shape of the lens surface in the horizontal scanning crosssection forms a non-arc shape, and this non-arc shape is expressed bythe following polynomial (1): $\begin{matrix}{X = {\left( {Y^{2}/{Rm}} \right)/\left\lbrack {{1 + \left. \sqrt{}\left\{ {1 - {\left( {1 + K} \right)\left( {Y/{Rm}} \right)^{2}}} \right\} \right. + {{A1} \cdot Y} + {{A2} \cdot Y^{2}} + {{A3} \cdot Y^{3}} + {{A4} \cdot Y^{4}} + {{A5} \cdot {A6} \cdot Y^{6}}},} \right.}} & (1)\end{matrix}$

wherein Rm denotes a paraxial radius of curvature in the horizontalscanning cross section, Y denotes a distance from the optical axis inthe horizontal scanning direction, K denotes a conical constant, A1, A2,A3, A4, A5, A6, . . . denote higher-order coefficients, and X denotes adepth in the direction of optical axis.

In the polynomial (1), when a numerical value other than zero issubstituted for coefficients A1, A3, A5, . . . of the odd order, thenon-arc shape becomes an asymmetric shape in the horizontal scanningdirection. In the first embodiment, since only the even order is used inthe polynomial (1), the non-arc shape becomes a symmetric shape in thehorizontal scanning direction.

The radius of curvature of the lens surface in the vertical scanningcross section is expressed by the following polynomial (2), when theradius of curvature in the vertical scanning cross section changes inthe horizontal scanning direction (expressed by coordinates (coordinatevalue Y) in which the optical axis is designated as an origin),$\begin{matrix}{{{{Cs}(Y)} = {{1/{{Rs}(0)}} + {{B1} \cdot Y} + {{B2} \cdot Y^{2}} + {{B3} \cdot Y^{3}} + {{B4} \cdot Y^{4}} + {{B5} \cdot Y^{5}} + \ldots}}\quad,} & (2)\end{matrix}$

wherein Rs(0) denotes a radius of curvature on the optical axis (Y=0) inthe vertical scanning cross section, B1, B2, B3, B4, B5, . . . denotehigher-order coefficients. In the polynomial (2), when a numerical valueother than zero is substituted for coefficients B1, B3, B5, . . . of theodd order, changes of the radius of curvature in the vertical scanningcross section become asymmetric in the horizontal scanning direction.

The secondary non-arc surface is expressed by the following equation 1.${X\left( {Y,Z} \right)} = {\frac{C_{m}Y^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right)C_{m}^{2}Y^{2}}}} + {\sum\limits_{n = 1}^{p}{a_{n}Y^{n}}} + \frac{{C_{s}(Y)}Z^{2}}{1 + \sqrt{1 - {\left( {1 + {K_{z}(Y)}} \right){C_{s}^{2}(Y)}Z^{2}}}} + {\sum\limits_{j = 1}^{r}{\left( {\sum\limits_{h = 0}^{q}{d_{j,h}Y^{h}}} \right)Z^{j}}}}$

The equation of the fourth term in this equation 1 is defined as thefollowing equation 2:${f_{SAG}\left( {Y,Z} \right)} = {\sum\limits_{j = 1}^{r}{\left( {\sum\limits_{h = 0}^{q}{d_{j,h}Y^{h}}} \right){Z^{j}.}}}$

This equation 2 can be broken down as shown in the following equation 3:f_(SAG)(Y, Z) = (F0 + F1 ⋅ Y + F2 ⋅ Y² + F3 ⋅ Y³ + F4 ⋅ Y⁴ + …  ) ⋅ Z + (G0 + G1 ⋅ Y + G2 ⋅ Y² + G3 ⋅ Y³ + G4 ⋅ Y⁴ + …  ) ⋅ Z² + (H0 + H1 ⋅ Y + H2 ⋅ Y² + H3 ⋅ Y³ + H4 ⋅ Y⁴ + …  ) ⋅ Z³ + (I0 + I1 ⋅ Y + I2 ⋅ Y² + I3 ⋅ Y³ + I4 ⋅ Y⁴ + …  ) ⋅ Z⁴ + (J0 + J1 ⋅ Y + J2 ⋅ Y² + J3 ⋅ Y³ + J4 ⋅ Y⁴ + …  ) ⋅ Z⁵ + …

In equations 1 to 3, Y denotes a position of the vertical scanning crosssection in the horizontal scanning direction (the coordinates in which aposition of the optical axis is designated as an origin), Z denotes thecoordinates in the vertical scanning direction, Cm or 1/Rm denotes aparaxial radius of curvature in the direction corresponding to thehorizontal scanning in the vicinity of the optical axis, Cs(0) or1/Rs(0) denotes a paraxial radius of curvature in the directioncorresponding to the vertical scanning in the vicinity of the opticalaxis, Cs (Y) denotes a paraxial radius of curvature in the directioncorresponding to the vertical scanning in the direction Y correspondingto the horizontal scanning, Kz(Y) denotes a conical constant expressinga quadratic surface in the direction corresponding to the verticalscanning in the direction Y corresponding to the horizontal scanning,and f_(SAG)(Y, Z) denotes a higher-order correction amount of anaspheric surface.

The paraxial radius of curvature Cs in the direction corresponding tothe vertical scanning is expressed by the following polynomial (3), andthe conical constant Kz expressing a quadratic surface in the directioncorresponding to the vertical scanning is expressed by the followingpolynomial (4):

Cs=1/Rs(0)+B 1·Y+B 2 ·Y ² +B 3·Y ³ +B 4·Y ⁴ +B 5·Y ⁵+ . . . ,  (3)

Kz=C 0+C 1·Y+C 2·Y ² +C 3·Y ³ +C 4·Y ⁴ +C 5·Y⁵+ . . .  (4)

In the polynomial (3), when a numerical value other than zero issubstituted for coefficients B1, B3, B5, . . . of the odd order of Y,changes of radius of curvature in the vertical scanning cross sectionbecomes asymmetric in the horizontal scanning direction.

Similarly, when a numerical value other than zero is substituted forcoefficients of the odd order of Y, expressing a non-arc amount, such asC1, C3, C5, . . . , F1, F3, F5, . . . , and G1, G3, G5, . . . , changesof the non-arc amount in the vertical scanning cross section becomesasymmetric in the horizontal scanning direction.

EXAMPLE 1

The size and arrangement of each component of the optical system in theoptical scanner in Example 1 will be explained with reference to FIG. 3and FIG. 4. FIG. 4 is a sectional view which shows a surfaceperpendicular to the optical axis (a surface perpendicular to the faceof paper) in the optical system shown in FIG. 3.

The light source 1 emits laser-beams having a wavelength of 780 nm. Thecoupling lens 2 has a focal distance of 27 mm, and has a collimateaction as the coupling action. The coefficient of linear expansion of anot-shown fitting section (base member) of the coupling lens 2 and thelight source 1 is 2.31×10⁻⁵.

The deflector 4 is formed of a polygon mirror, and the number ofdeflecting reflective surfaces thereof is 5, the radius of inscribedcircle thereof is 18 mm, and an angle formed by the incident directionof beams from the light source 1 and the optical axis of the scanningoptical system (the first and second scanning lenses 5 and 6) is 60degrees (see FIG. 3.

The lens 3 is formed of a glass lens having a positive power in thevertical scanning direction. The refractive index of the glass lens asthe lens 3 is 1.51119 at a temperature of 25° C., 1.51113 at atemperature of 45° C. and the coefficient of linear expansion of theglass lens is 7.5×10⁻⁶. The thickness d5 of the lens 3 (the distancebetween the plane 3 a and the plane 3 b on the optical axis) is 3 mm,and the distance d6 between the plane 3 b of the lens 3 and thereflective surface of the deflector 4 on the optical axis is 44.8 mm(see FIG. 4). The radius of curvature of the plane 3 a of the lens 3 is∞ in the horizontal scanning direction and 23 mm in the verticalscanning direction, and the radius of curvature of the plane 3 b thereofis ∞ (plane).

A scanning lens made of resin is used as the first scanning lens 5, anda scanning lens made of resin is used as the second scanning lens 6. Therefractive index of the resin scanning lens as the first scanning lens 5and the second scanning lens 6 is 1.523978 at a temperature of 25° C.,1.523088 at a temperature of 35° C., and 1.522197 at a temperature of45° C. The coefficient of linear expansion of the resin scanning lens asthe first scanning lens 5 and the second scanning lens 6 is 7×10⁻⁵.

The distance d7 between the reflective surface of the deflector 4 andthe plane 5 a of the first scanning lens 5 on the optical axis is 71.6mm, the thickness d8 of the first scanning lens 5 (the distance betweenthe plane 5 a and the plane 5 b on the optical axis) is 30 mm, thedistance d9 between the plane 5 b of the first scanning lens 5 and theplane 6 a (plane of incidence) of the second scanning lens 6 on theoptical axis is 66.3 mm, the thickness d10 of the second scanning lens 6(the distance between the plane 6 a and the plane 6 b on the opticalaxis) is 8.5 mm, the distance d11 between the plane 6 b (outgoing plane)of the second scanning lens 6 and the surface to be scanned 9 a on theoptical axis is 159.33 mm, the distance d12 is 0.2 mm and the distanced13 is 0.2 mm (see FIG. 4). The distance d12 and d13 is an amount ofshift given by shifting the optical axis of the first and the secondscanning lenses 5, 6 in the horizontal scanning direction in parallel,with respect to the optical axis.

Coefficients of the planes 5 a and 5 b of the first scanning lens 5, andthe planes 6 a and 6 b of the second scanning lens 6 in the horizontalscanning direction and in the vertical scanning direction are listed inTables 1 to 4, respectively. In Tables 1 to 4, Rm denotes a radius ofcurvature in the horizontal scanning direction, and Rs denotes a radiusof curvature in the vertical scanning direction. “E+02” denotes 10², and“E−08” denotes 10⁻⁸, and these figures affect the figure immediatelybefore.

TABLE 1 Plane 5a Rm = −1030.233346 Rs = −89.518927 Coefficients in theCoefficients in the horizontal scanning vertical scanning directiondirection A00 −4.041619E+02 B01 −9.317851E−06 A04 +6.005017E−08 B02+3.269905E−06 A06 −7.538155E−13 B03 +4.132497E−09 A08 −4.036824E−16 B04−4.207716E−10 A10 +4.592164E−20 B05 −1.170114E−12 A12 −2.396524E−24 B06+4.370640E−14 Blank B07 +2.347965E−16 B08 −6.212795E−18 B09−3.967994E−20 B10 −3.873869E−21 B11 +3.816823E−24 B12 +4.535843E−25

TABLE 2 Plane 5b Rm = −109.082474 Rs = −110.881332 Coefficients in theCoefficients in the horizontal scanning vertical scanning directiondirection A00 −5.427642E−01 B02 −3.652575E−07 A04 +9.539024E−08 B04+2.336762E−11 A06 +4.882194E−13 B06 +8.426224E−14 A08 −1.198993E−16 B08−1.026127E−17 A10 +5.029989E−20 B10 −2.202344E−21 A12 −5.654269E−24 B12+1.224555E−26

TABLE 3 Plane 6a Rm = +1493.654587 Rs = −70.072432 Coefficients in theCoefficients in the horizontal scanning vertical scanning directiondirection A00 +5.479389E+01 B02 −8.701573E−08 A04 −7.606757E−09 B04+2.829315E−11 A06 −6.311203E−13 B06 −1.930080E−15 A08 +6.133813E−17 B08+2.766862E−20 A10 −1.482144E−21 B10 +2.176995E−24 A12 +2.429275E−26 B12−6.107799E−29 A14 −1.688771E−30 Blank

TABLE 4 Plane 6b (Secondary non-arc surface) Rm = +1748.583900 Rs =−28.034612 Coefficients in the Coefficients in the horizontal scanningvertical scanning direction direction A00 −5.488740E+02 B01−1.440188E−06 A04 −4.978348E−08 B02 +4.696142E−07 A06 +2.325104E−12 B03+1.853999E−11 A08 −7.619465E−17 B04 −4.153092E−11 A10 +3.322730E−21 B05−8.494278E−16 A12 −3.571328E−26 B06 +2.193172E−15 A14 −2.198782E−30 B07+9.003631E−19 Blank B08 −9.271637E−21 B09 −1.328111E−22 B10−1.409647E−24 B11 +5.520183E−27 B12 +4.513104E−30 C00 −9.999999E−01 I00−1.320849E−07 I02 −1.087674E−11 I04 −9.022577E−16 I06 −7.344134E−20 K00+9.396622E−09 K02 +1.148840E−12 K04 +8.063518E−17 K06 −1.473844E−20

In the Example 1, a temperature distribution described below is obtainedby the heat source of the deflector 4 which rotates at a high speed.That is, T01=T02=T1=45° C., and T2=35° C. T01 is a temperature in thefirst optical system 21, T02 is a temperature in the second opticalsystem 22, T1 is a temperature in an atmosphere in the vicinity of thefirst scanning lens 5, and T2 is a temperature in an atmosphere in thevicinity of the second scanning lens 6. The definitions of T01, T02, T1and T2 apply similarly to Examples 2 to 4 described below.

Examples of results of changes in the curvature of field occurring inthe first to the third optical systems 21, 22 and 23 under theabove-described temperature distribution are shown in Table 5.

TABLE 5 Before taking measures (unit: mm) Examples (unit: mm) HorizontalVertical Horizontal Vertical scanning scanning scanning scanning Changes−0.43 −0.01 −0.43 −0.01 in curvature of field by first optical systemChanges 0.00 0.01 0.00 0.01 in curvature of field by second opticalsystem Changes 1.10 0.03 1.10 0.03 in curvature of field by firstscanning lens Changes 0.00 1.46 0.00 0.73 in curvature of field bysecond scanning lens Total 0.67 1.48 0.67 0.75

In Example 1, the lateral magnification of the whole optical systemformed of the first optical system 21, the second optical system 22 andthe third optical system 23 is such that the lateral magnification βm inthe horizontal scanning direction is 8.45, and the lateral magnificationβs in the vertical scanning direction is 1.55. In Table 5, sinceT01=T02=T1=T2=45° C. before taking measures, any temperaturedistribution does not occur in the optical housing.

In the first optical system 21, since the base member to which the lightsource 1 and the coupling lens 2 are fitted expands, the curvature offield in the horizontal scanning direction and in the vertical scanningdirection changes to the negative side at the time of temperature rise.However, since the lateral magnification of the whole optical system isset so that the relation |βm|>|βs| is realized, the amount of correctionof the curvature of field by means of the first optical system 21 issuch that the amount of correction is larger in the horizontal scanningdirection than in the vertical scanning direction. Therefore, it isnecessary to suppress in the vertical scanning direction the changes ofcurvature of field which occur in the third optical system 23. Hence, asdescribed above, by setting T1 (45° C.)>T2 (35° C.), changes incurvature of field of the whole optical system can be suppressed.

As described above, according to the Example 1, changes in curvature offield of the first to the third optical systems 21, 22 and 23 due to thetemperature change can be reduced, thereby the diameter of beams can bereduced and stabilized. Further, even if a multi-beam is used for thelight source 1, since the change in curvature of field in the verticalscanning direction is small, the pitch deflection of the verticalscanning beam pitch can be reduced.

EXAMPLE 2

In Example 2, the optical system in the optical scanner has aconfiguration such that, in the configuration of the optical system inthe optical scanner in Example 1 shown in FIG. 3, the coefficient oflinear expansion of the fitting section (base member), to which thecoupling lens 2 and the light source 1 are fitted, is changed from2.31×10⁻⁵ to 3.1×10⁻⁵, and the lens made of glass as the lens 3 of thesecond optical system 22 is changed to a lens made of resin.

The radius of curvature of the plane 3 a (plane of incidence) of thelens 3 formed of the resin lens is in the horizontal scanning directionand 23.57 mm in the vertical scanning direction, and the radius ofcurvature of the plane 3 b (outgoing plane) of the lens 3 is (plane).The refractive index of the resin lens is 1.523978 at a temperature of25° C., and 1.522197 at a temperature of 45° C. The coefficient oflinear expansion of the resin lens is 7×10⁻⁵.

Also in the Example 2, as in the Example 1, such a temperaturedistribution of T01=T02=T1=45° C., and T2=35° C. is obtained by the heatsource of the deflector 4 which rotates at a high speed. Examples ofresults of changes in the curvature of field occurring in the first tothe third optical systems 21, 22 and 23 under such a temperaturedistribution are shown in Table 6.

TABLE 6 Before taking measures (unit: mm) Examples (unit: mm) HorizontalVertical Horizontal Vertical scanning scanning scanning scanning Changes−0.73 −0.02 −0.73 −0.02 in curvature of field by first optical systemChanges 0.00 0.18 0.00 0.18 in curvature of field by second opticalsystem Changes 1.10 0.03 1.10 0.03 in curvature of field by firstscanning lens Changes 0.00 1.46 0.00 0.73 in curvature of field bysecond scanning lens Total 0.37 1.64 0.37 0.91

In the Example 2, the lateral magnification of the whole optical systemformed of the first optical system 21, the second optical system 22 andthe third optical system 23 is such that the lateral magnification βm inthe horizontal scanning direction is 8.45, and the lateral magnificationβs in the vertical scanning direction is 1.55. In Table 6, sinceT01=T02=T1=T2=45° C. before taking measures, any temperaturedistribution does not occur in the optical housing.

In the first optical system 21, since the base member to which the lightsource 1 and the coupling lens 2 are fitted expands, the curvature offield in the horizontal scanning direction and in the vertical scanningdirection changes to the negative side at the time of temperature rise.However, since the lateral magnification of the whole optical system isset so that the relation |βm|>|βs| is realized, the amount of correctionof the curvature of field by means of the first optical system 21 issuch that the amount of correction is larger in the horizontal scanningdirection than in the vertical scanning direction, and the curvature offield in the vertical scanning direction moves in the positive directionby the second optical system 22 (the change in the curvature of field is0.18 mm). Therefore, it is necessary to suppress in the verticalscanning direction the changes of curvature of field which occur in thethird optical system 23. Hence, as described above, by setting T1 (45°C.)>T2 (35° C.), changes in curvature of field of the whole opticalsystem can be suppressed.

As described above, according to the Example 2, the same working effectsas those of Example 1 can be obtained.

EXAMPLE 3

The size and arrangement of each component of the optical system in theoptical scanner in Example 3 will be explained with reference to FIG. 5and FIG. 7. The optical system in the optical scanner shown in FIG. 5has a configuration such that lenses 31 and 32 are added in the secondoptical system 22, in the configuration of the optical system in Example1 shown in FIG. 3. In FIG. 5, the parts which function in the samemanner as the optical system in the optical scanner shown in FIG. 3 aredenotes by the same Reference numerals. FIG. 6 is a sectional view whichshows a surface perpendicular to the optical axis (a surfaceperpendicular to the face of paper) in the optical system shown in FIG.5. FIG. 7 is an enlarged diagram in which the shape and arrangement ofthe lens 3, the lens 31 and the lens 32 of the second optical areenlarged.

In FIG. 7, the lens 31 is formed of a lens made of resin, having anegative power only in the vertical scanning direction. The lens 32 isformed of a lens made of glass, having a negative power in the verticalscanning direction.

The radius of curvature of the plane 31 a of the lens 31 is ∞ (plane),the radius of curvature of the plane 31 b thereof is ∞ in the horizontalscanning direction and 19.82 mm in the vertical scanning direction. Onthe other hand, the radius of curvature of the plane 32 a of the lens 32is and −18.7 mm in the vertical scanning direction, the radius ofcurvature of the plane 32 b thereof is 1.0E+8 in the horizontal scanningdirection and 18.03 mm (secondary non-arc surface) in the verticalscanning direction. The radius of curvature of the plane 3 a of the lens3 is ∞ and 13.54 mm in the vertical scanning direction, and the radiusof curvature of the plane 3 b thereof is (plane).

The refractive index of the resin lens as the lens 31 is 1.523978 at atemperature of 25° C., and 1.522197 at a temperature of 45° C. Thecoefficient of linear expansion of the resin lens as the lens 31 is7×10⁻⁵. Further, the refractive index of the glass lens as the lens 32is 1.51119 at a temperature of 25° C., and 1.51113 at a temperature of45° C. The coefficient of linear expansion of the glass lens as the lens32 is 7.5×10⁻⁶. The refractive index of the glass lens as the lens 3 is1.733278 at a temperature of 25° C., and 1.733058 at a temperature of45° C. The coefficient of linear expansion of the glass lens as the lens3 is 5.4×10⁻⁶.

In FIG. 6, the thickness d1 of the lens 31 on the optical axis (thedistance between the plane 31 a and the plane 31 b on the optical axis)is 3 mm, the distance d2 between the plane 31 b (outgoing plane) of thelens 31 and the plane 32 a (plane of incidence) of the lens 32 is 9.2mm, the thickness d3 of the lens 32 (the distance between the plane 32 aand the plane 32 b on the optical axis) is 3 mm, the distance d4 betweenthe plane 32 b (outgoing plane) of the lens 32 and the plane 3 a (planeof incidence) of the lens 3 is 8.15 mm, the thickness d5 of the lens 3(the distance between the plane 3 a and the plane 3 b on the opticalaxis) is 6 mm, and the distance d6 between the plane 3 b of the lens 3and the reflective surface of the deflector 4 on the optical axis is 144mm. In FIG. 6, respective values of d7 to d11 are the same as those ofd7 to d11 in the optical system in Example 1 shown in FIG. 4.

Coefficients of the plane 32 a of the lens 32 in the horizontal scanningdirection and in the vertical scanning direction are listed in Table 7.This plane 32 a has only a power in the vertical scanning direction. InTables 7, Rm denotes a radius of curvature in the horizontal scanningdirection, and Rs denotes a radius of curvature in the vertical scanningdirection. E+01 denotes 10¹, E−07 denotes 10⁻⁷, and these figures affectthe figure immediately before.

TABLE 7 Plane 32a Rm = +1.00E+08 Rs = 18.03 Coefficients in theCoefficients in the horizontal scanning vertical scanning directiondirection A04 +1.287048E−07 C00 +3.681387E+01 A06 +1.615827E−09 C02+1.882281E−01 Blank C04 +1.542188E−02 C06 −4.096661E−04 C08+5.584789E−06 I00 +3.496085E−04 I02 −2.319818E−06 I04 −7.859564E−08 I06+7.462640E−10 I08 −2.952126E−11 K00 +6.055635E−06 K02 −1.070845E−06 K04−1.078958E−07 K06 +2.023609E−09 K08 −2.307748E−11

In the Example 3, as in the Example 1, such a temperature distributionof T01=T02=T1=45° C., and T2=35° C. is obtained by the heat source ofthe deflector 4 which rotates at a high speed. Examples of results ofchanges in the curvature of field occurring in the first to the thirdoptical systems 21, 22 and 23 described above, under such a temperaturedistribution are shown in Table 8.

TABLE 8 Before taking measures (unit: mm) Examples (unit: mm) HorizontalVertical Horizontal Vertical scanning scanning scanning scanning Changes−0.43 −0.02 −0.43 −0.01 in curvature of field by first optical systemChanges 0.00 −0.75 0.00 −0.75 in curvature of field by second opticalsystem Changes 1.10 0.03 1.10 0.03 in curvature of field by firstscanning lens Changes 0.00 1.46 0.00 0.73 in curvature of field bysecond scanning lens Total 0.67 0.72 0.67 −0.01

In the Example 3, the lateral magnification of the whole optical systemformed of the first optical system 21, the second optical system 22 andthe third optical system 23 is such that the lateral magnification βm inthe horizontal scanning direction is 8.45, and the lateral magnificationβs in the vertical scanning direction is 1.55. In Table 7, sinceT01=T02=T1=T2=45° C. before taking measures, any temperaturedistribution does not occur in the optical housing.

As is clearly seen from Table 8, in the second optical system 21, thecurvature of field in the vertical scanning direction is corrected byusing a resin lens as the lens 31 having a negative power in thevertical scanning direction, but the correction is still insufficient.It is necessary to further increase the negative power of the resinlens, in order to allow the resin lens as the lens 31 to have furthercorrection function, thereby a problem in machining of the resin lensoccurs, and a wave front aberration is deteriorated. Therefore, bysetting T1 (45° C.)>T2 (35° C.) as described above, changes in curvatureof field of the whole optical system can be suppressed.

One example of beam spot diameters at each image height in the Example 3is shown in Table 9.

TABLE 9 Beam diameter Beam diameter (μm) in (μm) in horizontal verticalImage height (mm) scanning scanning 150 29.4 40.7 90 29.5 40.9 0 29.240.7 −90 29.5 40.9 −150 29.4 40.7

As described above, according to the Example 3, the same working effectsas those in Example 1 are obtained. In Example 3, since it is notnecessary to reduce the radius of curvature in the vertical scanningdirection of the resin lens as the lens 31 having a negative power, thewave front aberration is not deteriorated. As is clearly seen from Table9, excellent beam spot diameter can be obtained.

EXAMPLE 4

The optical system in the optical scanner in the Example 4 has aconfiguration such that, in the configuration of the optical system inthe optical scanner in Example 3 shown in FIG. 5, a negative power inthe horizontal scanning direction is added to the resin lens as the lens31, and a positive power in the horizontal scanning direction is addedto the glass lens as the lens 32.

In other words, the resin lens as the lens 31 has a negative power inthe horizontal scanning direction, and a positive power in the verticalscanning direction. The glass lens as the lens 32 has a positive powerboth in the horizontal scanning direction and in the vertical scanningdirection, and the glass lens as the lens 3 has a positive power in thevertical scanning direction.

The radius of curvature of the plane 31 a of the lens 31 is ∞ (plane),and the radius of curvature of the plane 31 b thereof is 150 mm in thehorizontal scanning direction and 19.82 mm in the vertical scanningdirection. The radius of curvature of the plane 32 a of the lens 32 is151 mm in the horizontal scanning direction and 150 mm in the verticalscanning direction. The radius of curvature of the plane 32 b of thelens 32, and the radius of curvature of the planes 3 a and 3 b of thelens 3 are the same as those in Example 3. The refractive index and thecoefficient of linear expansion of the lenses 31, 32 and 3 are the sameas those in Example 3. In FIG. 6, respective values of d1 to d11 are thesame as those in Example 3.

Also in the Example 4, as in the Example 1, such a temperaturedistribution of T01=T02=T1=45° C., and T2=35° C. is obtained by the heatsource of the deflector 4 which rotates at a high speed. Examples ofresults of changes in the curvature of field occurring in the first tothe third optical systems 21, 22 and 23 under such a temperaturedistribution are shown in Table 10.

TABLE 10 Before taking measures (unit: mm) Examples (unit: mm)Horizontal Vertical Horizontal Vertical scanning scanning scanningscanning Changes −0.40 −0.02 −0.40 −0.02 in curvature of field by firstoptical system Changes −0.77 −0.75 −0.77 −0.75 in curvature of field bysecond optical system Changes 1.10 0.03 1.10 0.03 in curvature of fieldby first scanning lens Changes 0.00 1.46 0.00 0.73 in curvature of fieldby second scanning lens Total −0.07 0.72 −0.07 −0.01

In the Example 4, the lateral magnification of the whole optical systemformed of the first optical system 21, the second optical system 22 andthe third optical system 23 is such that the lateral magnification βm inthe horizontal scanning direction is 8.45, and the lateral magnificationβs in the vertical scanning direction is 1.55. In Table 7, sinceT01=T02=T1=T2=45° C. before taking measures, any temperaturedistribution does not occur in the optical housing.

As is obvious from Table 10, in the second optical system 21, thecurvature of field in the vertical scanning direction is corrected byusing a resin lens as the lens 31 having a negative power in thevertical scanning direction, but the correction is still insufficient.It is necessary to further increase the negative power of the resinlens, in order to allow the resin lens as the lens 31 to have furthercorrection function, thereby a problem in machining of the resin lensoccurs, and a wave front aberration is deteriorated. Therefore, bysetting T1 (45° C.)>T2 (35° C.), as described above, changes incurvature of field of the whole optical system can be suppressed, andchanges in curvature of field in the vertical scanning direction of thewhole optical system can be suppressed.

In the Example 4, the resin lens as the lens 31 is only one, but theresin lens may be two. In this case, the two resin lenses may be acombination of a lens having only a negative power in the verticalscanning direction and a lens having only a power in the horizontalscanning direction, or the both lenses may have a negative power both inthe vertical scanning direction and in the vertical scanning direction.

As explained above, according to the Example 4, the same working effectsas those in Example 1 can be obtained.

FIG. 8 is a block diagram of the configuration of the main parts of animage formation apparatus according to a second embodiment of thepresent invention. In the image formation apparatus shown in FIG. 8, acharger 42, an optical scanner 43, a developing device 44, a transfercharger 45, a fixing device 46, and a cleaning device 47 are arrangedaround an image supporting body 41. The optical scanner 43 is, forexample, the optical scanner in either one of Examples 1 to 4 in thefirst embodiment.

There is an electrophotographic process as a representative one of theimage formation process for forming an image. In the electrophotographicprocess, an image is formed by a series of processes, such that a lightbeam spot from the optical scanner 43 is irradiated onto aphotosensitive material as the image supporting body 41 uniformlycharged by the charger 42, to form a latent image (exposure), a toner isadhered on the latent image by the developing device 44 to form a tonerimage (development), and the toner image is then transferred onto arecording paper S by the transfer charger 45 (transfer), and fused onthe recording paper S by applying pressure and heat by the fixing device46 (fixation).

As explained above, according to the second embodiment, high qualityimage can be obtained, by using the optical scanner as an exposure unitof the image formation apparatus.

According to the first aspect of the present invention, the opticalscanner of the invention comprises a first optical system having acoupling lens, a second optical system which is formed of a glass lenshaving a positive power in the vertical scanning direction, or a resinlens having a positive power in the vertical scanning direction, andforms the light flux from a light source, having passed through thefirst optical system, into a line image extending in the horizontalscanning direction, on a deflector, and a third optical system which hasa first optical element made of resin, having a positive power in thehorizontal scanning direction, and a second optical element made ofresin, having a positive power in the vertical scanning direction, andwhich condenses the light flux deflected by the deflector as an opticalbeam spot on a surface to be scanned. An absolute value of a lateralmagnification in the horizontal scanning direction of the whole opticalsystem, which is formed of the first optical system, the second opticalsystem, and the third optical system, is set larger than an absolutevalue of a lateral magnification in the vertical scanning direction, andthe temperature distribution generation unit makes it possible tosatisfy the relation: the temperature near the first optical element T1is greater than the temperature near the second optical element T2. As aresult, changes of beam diameter of the optical beam due to atemperature change and a pitch deflection of a vertical scanning beampitch of multi-beam can be reduced, thereby making it easy to have themulti-beam. Hence, the number of rotation of the deflector such as apolygon scanner can be reduced, to thereby provide an optical scannerthat can realize high durability, low noise and low power consumption.

According to the second aspect of the present invention, the opticalscanner comprises a first optical system having a coupling lens thatcouples a light flux from a light source, a second optical system thatforms the light flux into a line image extending in the horizontalscanning direction, on a deflector, the second optical system includinga glass lens having a positive power in a vertical scanning directionand a resin lens having a negative power in the vertical scanningdirection, a third optical system having a scanning image formationdevice that condenses the light flux deflected by the deflector as anoptical beam spot on a surface to be scanned, the third optical systemincluding a first optical element made of resin, having a positive powerin the horizontal scanning direction, and a second optical element madeof resin, having a positive power in the vertical scanning direction,and a temperature distribution generation unit which controls anatmospheric temperature T1 near the first optical element and anatmospheric temperature T2 near the second optical element such thatT1>T2. Moreover, an absolute value of a lateral magnification in thehorizontal scanning direction of an optical system, which is formed ofthe first optical system, the second optical system, and the thirdoptical system, is set larger than an absolute value of a lateralmagnification in the vertical scanning of the optical system.

Moreover, the second optical system includes a resin lens having anegative power both in the horizontal scanning direction and in thevertical scanning direction and a plurality of glass lenses, at leastone of those having a positive power in the vertical scanning direction.Hence, changes of beam diameter due to a temperature change and a pitchdeflection of a vertical scanning beam pitch of multi-beam can bereduced. Further, by satisfying the relation T1>T2, the negative powerin the vertical scanning direction of the resin lens can be decreased.The beam diameter can be also decreased, by obtaining excellent wavefront aberration.

Furthermore, by using the optical scanner according to any one of thefirst to third aspects, an image formation apparatus which can outputhigh quality images which are stable and excellent in granularity,resolution and gradient can be provided.

The present document incorporates by reference the entire contents ofJapanese priority document, 2001-287766 filed in Japan on Sep. 20, 2001and 2002-271602 filed in Japan on Sep. 18, 2002.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. An optical scanner comprising: a first opticalsystem having a coupling lens that couples a light flux from a lightsource; a second optical system that forms the light flux into a lineimage extending in the horizontal scanning direction, on a deflector,the second optical system including a glass lens having a positive powerin a vertical scanning direction, or a resin lens having a positivepower in the vertical scanning direction; a third optical system havinga scanning image formation device that condenses the light fluxdeflected by the deflector as an optical beam spot on a surface to bescanned, the third optical system including a first optical element madeof resin, having a positive power in the horizontal scanning direction,and a second optical element made of resin, having a positive power inthe vertical scanning direction; and a temperature distributiongeneration unit which controls atmospheric temperature T1 near the firstoptical element, and atmospheric temperature T2 near the second opticalelement such that T1>T2, wherein an absolute value of a lateralmagnification in the horizontal scanning direction of an optical system,which includes the first optical system, the second optical system, andthe third optical system, is set larger than an absolute value of alateral magnification in the vertical scanning of the optical system. 2.The optical scanner according to claim 1, wherein the second opticalsystem includes a resin lens and a plurality of glass lenses, the resinlens has a negative power both in the horizontal scanning direction andin the vertical scanning direction, and at least one of the glass lenseshas a positive power in the vertical scanning direction.
 3. An opticalscanner comprising: a first optical system having a coupling lens thatcouples a light flux from a light source; a second optical system thatforms the light flux into a line image extending in the horizontalscanning direction, on a deflector, the second optical system includinga glass lens having a positive power in a vertical scanning direction,and a resin lens having a negative power in the vertical scanningdirection; a third optical system having a scanning image formationdevice that condenses the light flux deflected by the deflector as anoptical beam spot on a surface to be scanned, the third optical systemincluding a first optical element made of resin, having a positive powerin the horizontal scanning direction, and a second optical element madeof resin, having a positive power in the vertical scanning direction;and a temperature distribution generation unit which controlsatmospheric temperature T1 near the first optical element, andatmospheric temperature T2 near the second optical element such thatT1>T2, wherein an absolute value of a lateral magnification in thehorizontal scanning direction of an optical system, which includes thefirst optical system, the second optical system, and the third opticalsystem, is set larger than an absolute value of a lateral magnificationin the vertical scanning of the optical system.
 4. An image formationapparatus comprising an optical scanner, the optical scanner including afirst optical system having a coupling lens that couples a light fluxfrom a light source; a second optical system that forms the light fluxinto a line image extending in the horizontal scanning direction, on adeflector, the second optical system including a glass lens having apositive power in a vertical scanning direction, or a resin lens havinga positive power in the vertical scanning direction; a third opticalsystem having a scanning image formation device that condenses the lightflux deflected by the deflector as an optical beam spot on a surface tobe scanned, the third optical system including a first optical elementmade of resin, having a positive power in the horizontal scanningdirection, and a second optical element made of resin, having a positivepower in the vertical scanning direction; and a temperature distributiongeneration unit which controls atmospheric temperature T1 near the firstoptical element, and atmospheric temperature T2 near the second opticalelement such that T1>T2, wherein an absolute value of a lateralmagnification in the horizontal scanning direction of an optical system,which includes the first optical system, the second optical system, andthe third optical system, is set larger than an absolute value of alateral magnification in the vertical scanning of the optical system. 5.An image formation apparatus comprising an optical scanner, the opticalscanner including a first optical system having a coupling lens thatcouples a light flux from a light source; a second optical system thatforms the light flux into a line image extending in the horizontalscanning direction, on a deflector, the second optical system includinga glass lens having a positive power in a vertical scanning direction,and a resin lens having a negative power in the vertical scanningdirection; a third optical system having a scanning image formationdevice that condenses the light flux deflected by the deflector as anoptical beam spot on a surface to be scanned, the third optical systemincluding a first optical element made of resin, having a positive powerin the horizontal scanning direction, and a second optical element madeof resin, having a positive power in the vertical scanning direction;and a temperature distribution generation unit which controlsatmospheric temperature T1 near the first optical element, andatmospheric temperature T2 near the second optical element such thatT1>T2, wherein an absolute value of a lateral magnification in thehorizontal scanning direction of an optical system, which includes thefirst optical system, the second optical system, and the third opticalsystem, is set larger than an absolute value of a lateral magnificationin the vertical scanning of the optical system.